The present invention relates to the production of precipitated calcium carbonate (pcc) having a selected crystal form and products produced thereby. In particular, the invention relates to the preparation of pcc products having a coarse crystalline form. Such products are useful in matte paper grades, i.e. grades in which the paper surface has a dull, low-gloss finish.
Manufacture of pcc products and the use of such products in compositions to make or coat paper are well known. However, there can be considerable variation in the properties of the pcc product depending upon the process conditions employed to prepare the product.
PCC products which are coarse i.e. have a mean particle size greater than 1.5 .mu.m are known in the prior art, e.g. as described in U.S. Pat. No. 3,320,026, U.S. Pat. No. 4,882,310 wherein the particles are prismatic form and use of such coarse products in compositions to provide matte paper grades is also well known, e.g. from TAPPI Proceedings of 1982 Coating Conference, pages 143-146, "The influence of particle pigment shape on the performance of a paper coating, by Crenshaw; "Pigments for Matt Coated Papers" by Dr Billy Allwood, paper presented at PIRA Coated Paper Development Seminar held Sep. 13, 1990, Bolton, United Kingdom, U.S. Pat. No. 4,714,603 and U.S. Pat. No. 5,643,415 (WO9221613).
U.S. Pat. No. 4,714,603 describes a process for making such a coarse pcc product. The well known carbonation of slaked lime slurry using carbon dioxide is employed in the process described. A polyphosphate additive is included in the lime slurry to facilitate formation of coarse pcc particles. Carbonation is begun at a temperature of at least 15.degree. C., preferably from 30.degree. C. to 35.degree. C. The particle form obtained is a spherical cluster or aggregate of fine nodules, e.g. containing up to 5,000 such nodules, the nodules having a height of from 0.02 .mu.m to 1.0 .mu.m.
U.S. Pat. No. 5,643,415 describes production of a coarse pcc and use of the same in coating paper to give a dull (matte) finish in the manner described in the earlier prior art specified above. The pcc crystal form is described as prismatic but the form illustrated in the Figures of the patent is roughly spheroidal rather than what is understood in the art to be prismatic. Polyphosphate, sodium hexametaphosphate, is added to a basic calcium carbonate (lime/pcc) slurry to assist crystal nucleation.
It is an object of the present invention to provide an improved process for producing a coarse pcc product for use in coating matte grade papers.
It is a further object of the present invention to provide an improved, selected pcc product form for use in coating matte grade papers and to provide a method of producing such a product form.
These and further objects which will become apparent from the following description are met by the present invention.